In general, acrylic acid is produced by a two-stage oxidation reaction of undergoing catalytic reaction of propylene with oxygen in the presence of a catalyst to produce acrlolein and undergoing catalytic reaction of the resulting acrlolein with oxygen.
In the recent years, due to a price difference between propane and propylene and for the purpose of overcoming problems such as complexity in the process involving the two-stage oxidation, a process for producing acrylic acid in one stage by using propane as a starting raw material is studied, and there have been made a number of proposals with respect to catalysts to be used therefor. As representative examples thereof, there are enumerated catalysts made of a composite metal oxide such as [V, P, Te] bases, [Mo, Te, V, Nb] bases, and [Mo, Sb, V, Nb] bases.
Recently, there have been filed some applications for patent with respect to further improvements of the foregoing metal oxide catalysts. JP-A-10-137585 discloses a process for producing a catalyst by mixing an aqueous solution obtained by allowing a molybdenum compound, a vanadium compound and an antimony compound to react in an aqueous medium at 70° C. or higher with a niobium compound, evaporating to dryness the resulting mixture, and further calcining it at a high temperature.
JP-A-10-230164 describes that in heating the respective metal compounds in the aqueous medium described in the foregoing JP-A-10-137585, a gas containing molecular oxygen is introduced into the aqueous medium. Furthermore, it is described that when the catalyst as produced in this process is used for vapor phase catalytic oxidation reaction of propane, the yield of acrylic acid is enhanced.
Also, JP-A-11-285636 describes that hydrogen peroxide is added to a reaction liquid of the respective metal compounds in the aqueous medium during heating and reaction, or after the reaction as described in the foregoing JP-A-10-137585 and JP-A-10-230164.
However, even in the case of using the catalysts as described in all of the foregoing patent documents, the yield of acrylic acid as obtained in one-stage oxidation of propane does not reach a practical level required in the acrylic acid production.
JP-A-11-226408 discloses a method for allowing a metallic tellurium power to react with an oxonic acid salt (oxometalate) of other element, such as ammonium metavanadate, molybdic acid, and ammonium paramolybdate, in an aqueous medium, thereby dissolving the subject metal powder in a reaction liquid and using the resulting reaction liquid as a raw material of a metal oxide catalyst. However, the particle size of the tellurium particle to be used in the invention as described in the subject patent document exceeded 100 μm, and a long period of time was required for the reaction in the aqueous medium. Furthermore, when the resulting metal oxidation catalyst is used in ammoxidation of propane, it is possible to produce acrylonitrile in a propane conversion of from 25 to 30% and a selectivity of approximately 60%. However, when used for the reaction for producing acrylic acid from propane, there was no drastic advance in performance as compared with conventionally known catalysts.